I have one 23" shelf I mistakenly purchased. Everything else is 19".
I actually prefer the round hole unthreaded kind of rack. Not the square hole, nor the kind with threads on the rack rails themselves. The threaded kind can strip. But at least then you can add a cage nut to them. The square ones are a pain in the ass to add the nuts into the holes when you've got a full rack. Or when you want to move things around without tearing everything apart. Yeah, you generally shouldn't do it that way... With the round cage nut kind you can slide a nut up/down a position or two without pulling the gear out. Word of advice, buy double the amount of cage nuts you think you'll need... you'll still come up short.
Also note there are different kinds of screws. There are even some that have a post that screws in and a knurled nut on the face. This way you can screw in the post and have a point on which to rest the gear while muscling it into place. Just be sure to use the SAME thread size, whichever style you chose.
Word of advice, put anything that might require service on sliding rails. Yes, the rails can be pricey. But the ease with which you can service stuff far offsets the hassle of having to pull gear totally out of the rack whenever you need to change something in it. As in, computers that might be getting upgrades. There are a variety of schemes for dealing with the wires. Some are 'less worse' than others but they're all a bit of an adventure.
For items less likely to get altered, or ones that have hard connections to in-wall wiring, you're probably better off not putting them on sliding rails. This so you don't wreck the wiring and connectors.